The present invention relates to broadband ultraviolet light sources used in science and industry, and in particular, to a fiber-optic ultraviolet light source.
Broad-spectrum ultraviolet light sources coupled through a fiber-optic can be used for a wide variety of sensing and analysis applications. Normally, the source for a broad-spectrum ultraviolet light is an arc lamp in which gas is energized between opposed electrodes. This light source is relatively bulky, requires a high voltage, and is difficult to couple to a small diameter fiber-optic cable.
Non-ultraviolet broad-spectrum light sources using fiber-optics have been obtained by exciting fibers with narrow frequency light sources in a nonlinear mode as described in U.S. patent application Ser. No. 10/637,472 filed Aug. 8, 2003 assigned to the present inventors, and hereby incorporated by reference. In the invention of this previous application, a number of nonlinear mechanisms including: self-phase modulation, four-photon mixing, and stimulated Raman scattering act in concert to expand the spectrum.
As one moves to the shorter wavelengths of ultraviolet, the ability to sustain these nonlinear, frequency-spreading phenomenon is hampered by the high material dispersion of the fibers which tends to separate the frequencies of the pulse as a result of their different propagation speeds through the fiber. This pulse “walk off” suppresses the nonlinear effects substantially limiting the spectrum broadening.
Some success in creating a broad-spectrum fiber-optic source having short wavelengths in visible ranges has been obtained using two-stage process in which a first narrow-band laser such as a nitrogen laser excites a wide-band dye laser, the latter providing a broad-spectrum excitation pulse to the fiber-optic. A broad excitation pulse has been thought necessary to prevent obtaining a set of discrete “Stokes lines” that do not form a broad-spectrum-continuum when a narrow-band laser is used. Further, because as one moves to the ultraviolet region, light attenuation and phase dispersion becomes more severe, it is not clear whether nonlinear, frequency-spreading phenomenon can be used to create practical spectral broadening below a 400 nanometer wavelength.